diff --git a/.github/workflows/matrix-test.yml b/.github/workflows/matrix-test.yml
index 9553cf4820..08ac4dc526 100644
--- a/.github/workflows/matrix-test.yml
+++ b/.github/workflows/matrix-test.yml
@@ -48,7 +48,7 @@ jobs:
 
       - name: 'Determine number of cores to build on (Windows)'
         if: runner.os == 'Windows'
-        run: echo NPROC=%NUMBER_OF_PROCESSORS% >> $GITHUB_ENV
+        run: echo NPROC="${NUMBER_OF_PROCESSORS-unknown}" >> $GITHUB_ENV
 
       - name: 'Determine number of cores to build on (macOS)'
         if: runner.os == 'macOS'